(1) Field of the Invention
The present invention relates to volatility-inhibited fertilizers comprised of granular urea coated with a coating including a binding agent having a boron anion and a hydrogen bonding group to adhere said binding agent to the granular urea, and optionally with additional plant nutrients, and to process for their preparation.
(2) Description of the Prior Art
Urea is widely employed as a non-burning nitrogen source for agricultural and forest fertilization. The granular form is commonly used for forest fertilization whereas granular and liquid forms may be used for agricultural fertilization. When applied to the soil, the granular form of urea dissolves by absorbing water and entering the soil solution. The water may come from rain or irrigation, atmospheric moisture and from water in the soil (soil moisture).
Once in the soil solution, urea is subject to hydrolysis by the enzyme urease. The hydrolysis reaction may ultimately produce ammonia as indicated in equation 1.
H2NCONH2+H2Oxe2x86x922NH3+CO2 (1) 
The actual hydrolysis process proceeds through intermediates of ammonium carbamate and ammonium carbonate (see reviews by Terman xe2x80x9cVolatilization Losses of Nitrogen as Ammonia From Surface Applied Fertilizers, Organic Amendments, and Crop Residuesxe2x80x9dxe2x80x94Adv. Agronomy 31:189-223, 1979 and Freney et. al.xe2x80x94xe2x80x9cVolatilization of Ammoniaxe2x80x9d in Gaseous Loss of Nitrogen from Plant Soil-Systems Freney and Simpson editors, Martinus and Nijhoff, 1983). The volatilization problem with fertilizers has been the subject of much study and when urea is the fertilizer nitrogen source applied the governing factors are the ability of a given soil to release ammonia and the activity of the enzyme urease.
The loss of ammonia nitrogen from urea nitrogen is governed by the relationship between the ammonia/ammonium ion equilibrium and a number of soil variables. The soil variables include: temperature, soil pH, soil chemistry (cation exchange capacity and organic matter), and soil moisture. Increases in temperature favor ammonia release by increasing the base dissociation constant of ammonia and reducing the solubility of ammonia in the soil solution. An alkaline soil pH favors ammonia release by increasing the equilibrium percent of ammonia present in the soil solution. Soils with a high cation exchange capacity are better able to absorb ammonium ions reducing volatilization losses. Organic matter can absorb ammonia reducing losses as bacteria convert the ammonia to organic nitrogen. Applying urea or ammonia forming compounds to damp soils which are drying out due to wind or sunlight can increase ammonia losses.
The activity of the enzyme urease in a given soil is affected by temperature, soil pH, and the amount of enzyme present and dilution of the urea as it dissolves. Increases in temperature favor urea hydrolysis by increasing the activity of the enzyme urease. An alkaline soil pH also increases the activity of the enzyme urease. Organic matter is a significant source of the enzyme urease, which increases ammonia losses from urea. Urea applied under low moisture conditions takes longer to dissolve before hydrolysis can begin. Rainfall washes urea into the soil and dilutes the compound, which helps to reduce volatility losses.
Several methods have been used to reduce nitrogen losses from surface applied granular urea. Acidic coatings have been used to control the ammonia/ammonium ion equilibrium in the area where the particle dissolves. Urease inhibitors have been developed to reduce the activity of the enzyme urease, thus reducing volatile nitrogen losses. Finally, expensive, slow release urea compositions can be formed which extend the time needed to release the urea nitrogen.
As illustrative of acidic coatings, Young (U.S. Pat. No. 4,073,633; Feb. 14, 1978) teaches the use of an acid generating substance to keep the soil pH at 7 or less in the area where granular urea is applied. The acid generating substances include acids (inorganic and organic); salts such as ferric sulfate that are acidic in water; and compounds such as sulfur which are metabolized by soil microorganisms to acid compounds or form acidic oxidation products with oxygen. The products needed to exploit the invention can be made by coating urea granules or forming agglomerates of urea and the acid generating substances (e.g. the agglomerate indicated of urea, gypsum and an acid generating compound). The invention, also, indicates that clays such as kieselguhr in the range of 5% to 20% can be used to prevent sticking of coated products.
Whitehurst et. al. (U.S. Pat. No. 6,030,659; Feb. 29, 2000) teaches the formation of phosphate coatings on the surface of a urea granule by first reacting urea with an acid then allowing the acid surface to react with an insoluble phosphate mineral. The reaction with the insoluble phosphate mineral causes the formation of a soluble phosphate salt on the urea surface. The salts formed on the urea surface are typically acidic and help to reduce the volatile nitrogen losses.
Products having acidic coating materials are potentially corrosive to some metals used in fertilizer application equipment when damp. In addition, some micronutrients such as boron are unusable in aqueous acids due to low solubilityxe2x80x94e.g. boric acid forms a suspension in phosphoric acid which is difficult to use to form coated products. In addition to the difficulty of handling the boric acid suspension, coated products produced with the suspensions can be sticky with poor flow characteristics.
A number of compounds are known to inhibit urease. Examples are the benzoquinones (Anderson GB 1,142,245; Feb. 5, 1969); dithiocarbamates (Tomlinson GB 1,094,802xe2x80x94Dec. 13, 1967 and Hyson U.S. Pat. No. 3,073,694xe2x80x94Jan. 15, 1963); urea derivatives such as methylurea or thiourea (Sor et. al. U.S. Pat. No. 3,232,740; Feb. 1, 1966); phosphoric triamides (Kolc et. al. U.S. Pat. No. 4,530,714; Jul. 23, 1985); and organic bromine compounds and organic nitrates (Norden et. al. (U.S. Pat. No. 4,576,625; Mar. 18, 1986). Many of these compounds are expensive to use and some can be highly toxic.
In addition to the compounds identified above, metal ions and boron containing salts have been studied as urease inhibitors. Tabatabi (Soil Biology and Biochemistry 9:9-13, 1977) reported on the inhibition of soil urease by various metal ions and other compounds. All soils except one displayed less than 30% inhibition due to sodium tetraborate when used at a rate of 5 mole per gram soil. The exception was the Waller soil which had the lowest pH and lowest urease activity of the soils used in study.
Sor (U.S. Pat. No. 3,388,989; Jun. 18, 1968) discloses the formation of granules containing urea, a urease inhibitor and a hydrocarbon binder. The urease inhibitors revealed include soluble metal salts (Ag, Co, Cu, Hg, Mn, Mo, Pb), soluble borate salts, soluble metal fluorides and formaldehyde. The hydrocarbon binder includes waxes and asphalt. The urea, urease inhibitor and the heat softened hydrocarbon binder are mixed to form a granule. Sor indicates that it is desirable to heat all the ingredients when preparing the fertilizer mixture.
Sor et. al. (U.S. Pat. No. 3,565,599; Feb. 23, 1971) teaches the use of a urease inhibitor such as an alkali metal borate (sodium tetraborate) or boric acid in combination with a hydrophobic substance to reduce the nitrogen loss from urea fertilizers when applied to the soil. The boron source and the hydrophobic substance are preferentially distributed in the urea melt prior to prilling. The hydrophobic substances can include: waxes, vegetable oils, oleyl ether, polyethylene glycol, N-tallow trimethylene diamine, calcium petroleum sulfonate, naphthalene spray oils, octadecylamine and dimethylpolysioxane. The patent, also, claims a coating of octadecyl amine and sodium tetraborate coated unto urea.
Geissler (U.S. Pat. No. 3,523,018; Aug. 4, 1970) discloses the formation of urea granules containing urease inhibitors. The granules are formed by incorporating the inhibitor into the urea melt prior to prilling. The inhibitors claimed include: copper formate, copper acetate, aliphatic amines, boron trifluoride, alkyldithiocarbamates, hydroxylamine and a mixture of borax and copper sulfate. The patent discloses a number of other inhibitors that include: copper sulfate, borax, boric acid, EDTA copper complexes, copper tetrafluoroborate, and metal ions such as Cu, Co, Mn, Zn, fluorides, bromides and cyanides.
Van der Puy et. al. (U.S. Pat. No. 4,462,819; Jul. 31, 1984) discloses a number of urease inhibitors involving organic boron compounds. The basic general structure R1R2BOH where there is at least 1 carbonxe2x80x94boron bond. If there are two OH groups attached to the boron atom, then the carbonxe2x80x94boron bond will be to a substituted aromatic ring system.
The literature referenced above indicates that considerable interest exists for the development of compositions, which are able to inhibit urease. Some materials identified as inhibitors could not be used for fertilizers (Pb, Hg etc.) due to plant toxicity and other environmental issues. Some are too expensive for routine use. Some require special solvents for dispersion onto granular urea.
The process of coating a fertilizer particle with other materials using a binder is well known and can be viewed as a special case of agglomeration technologies which have been reviewed by Pietsch (See Wolfgang Pietschxe2x80x94Agglomeration Processes Phenomena, Technologies, Equipmentxe2x80x94Wiley VCH 2002 particularly pages 35-46; 151-187; 415-440).
Successful agglomeration (and/or coating) requires a suitable binding agent to hold the other powdered materials on the surface or to bind the agglomerate together when a low pressure method is used.
DiCicco (U.S. Pat. No. 3,560,192; Feb. 2, 1971) deals with the coating of granular fertilizers with micronutrients using an aqueous zinc chloride solution as a binder. The micronutrients must be in powdered form (less than 149 m). The fertilizer materials indicated are diammonium phosphate, ammonium nitrate, granular triple super phosphate and potassium chloride or mixture of these fertilizer materials.
Hall (U.S. Pat. No. 1,977,628; Oct. 23, 1934) discloses two methods for preparation of urea fertilizers containing other fertilizer materials. In the first method a water insoluble ballast material is added to a urea melt. The water insoluble ballast includes: rock phosphate, chalk, gypsum (calcium sulfate dihydrate). The melt may also contain potassium salts or ammonium phosphates. In the second method, the desired fertilizer materials in finely divided form are mixed together then coagulated into a granule. The coagulation process requires water or steam.
Miller (U.S. Pat. No. 3,961,932; Jun. 8, 1976) describes the coating of fertilizers with chelated micronutrients is described. The chelating agents described are mostly aminopolycarboxylates (EDTA family). The fertilizer to be treated is sprayed with a solution containing the chelated metal complex (1%-5% typically). The coating process is finished by adding a drying agent such as calcium silicate or silica. The drying agent must be in finely divided form.
Nau (U.S. Pat. No. 3,353,949; Nov. 21, 1967) teaches the formation of granular fertilizers containing micronutrients. The granular products are formed by mixing a substrate fertilizer particle (larger than 841 xcexcm in size) with the desired powdered micronutrients (less than 149 xcexcm in size) then adding a water-soluble conditioning agent. The conditioning agent is a 30% to 70% aqueous solution of sugars (glucose, dextrose, and molasses), alkali metal lignin sulfonates, or water-soluble fertilizers (ammonium nitrate or urea). In an alternate method indicated by Nau, the base fertilizer can be mixed with the conditioning agent then mixed with the powdered micronutrients.
Philen et. al. (U.S. Pat. No. 3,423,199; Jan 21, 1969) describes the coating of a fertilizer with pulverized micronutrients (Zn, Cu, Fe, Mn, Co, B, Mg, Mo, S, and mixtures). An in-situ macronutrient solution is formed on the surface of an existing fertilizer particle with water or steam. To the wetted surface powdered micronutrients are added. In a related patent Philen et. al. (U.S. Pat. No. 3,523,019; Aug. 4, 1970) discloses the use of an ammonium polyphosphate solution as a a binder.
Walter et. al. (AU 9645576; Sep. 5, 1996) teaches the formation of micronutrient coated urea products by the agglomeration of urea, the desired micronutrient using a coating additive to cause the micronutrients to stick to the urea. The coating process for urea involves the tumbling of urea with minute traces of water and particulate micronutrients or secondary nutrients or coating additives, which assist in binding the desired, other nutrients.
Detroit (U.S. Pat. No. 5,041,153; Aug. 20, 1991) teach es the use of lignin sulfonate salts to control caking and dusting in various fertilizers. The fertilizers covered are ammonium phosphates, calcium phosphates, sulfates, nitrates, and potassium chloride.
Buchholz (CA 1,337,460; Sep. 29, 1989) teaches the use of an a queous solution containing urea and lignin sulfonate to reduce tendency for dust formation of granular fertilizers. The urea-lignin sulfonate mixture is intended to cause small particles present to adhere to the larger particles present.
The need for conditioning agents for some fertilizers is well known. Sawyer et. al. (U.S. Pat. No. 3,234,003; Feb. 8, 1966) discloses the coating of fertilizer particles (high nitrogen content) with diatomaceous earth, various clays and limestone to prevent caking. The invention of Sawyer et. al. describes an additional conditioning agent composed of kaolin clay coated with an aliphatic amine (8-22 carbons) and a solvent for the amine consisting of a fatty nitrile (12-20 carbons) and a hydrocarbon oil. The patent then describes the use of the conditioning agent to treat hygroscopic fertilizer granules.
Van Hijfte et. al. (U.S. Pat. No. 4,500,336; Feb. 19, 1985) indicates that a composition composed of urea granules and super phosphate granules (single or triple) form a deliquescent mixture and describes the substitution reaction between urea and a super phosphate that results in the release of hydration water of the super phosphate. The invention describes the use of a crystallization inhibitor (aluminum salts) to prevent the reaction of urea with super phosphates.
Barry et. al. (U.S. Ser. No. 3,425,819; Feb. 4, 1969) describe the production of urea containing granules by spraying a bed of granular urea and recycled fines with a aqueous slurry of mono- and diammonium phosphates. The product granules are dried at temperatures from 140xc2x0 F. to 200xc2x0 F. Compositions which contain significant amount of DAP combined with urea will not provide volatility reductions due to the alkalinity of DAP.
Whitaker et. al. (U.S. Pat. No. 2,074,880) describes a molecular addition compound of urea and calcium sulfate prepared from urea and gypsum.
Young (U.S. Pat. No. 4,701,555; Oct. 20, 1997) describes a method for removing biuret from urea fertilizers which have been heated to 130xc2x0 C. or higher during manufacturing of the urea granules. Young indicates that biuret is a material toxic to plants.
The present invention relates to compositions to reduce volatilization losses from granular urea when used as a soil applied fertilizer, and in particular to granular urea coated with amino alcohol borates (borate and polyborate mixtures), resulting in coated urea products having reduced volatile nitrogen losses when applied to soils. In addition, it has been found that coated urea products prepared from aqueous solutions of borates containing complex ions of some divalent metals with amino alcohols (alkanolamines) will reduce volatile nitrogen losses when applied to soils. Numerous coated urea compositions can be made using the volatility inhibiting aqueous borate solutions allowing compositions to be altered to meet site specific requirements to control volatility losses from the included urea and to provide needed additional macronutrients (nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur) and/or micronutrients (boron, chlorine, copper, iron, manganese, molybdenum, nickel, and zinc) required on some soils. The terms macronutrient and micronutrient follow the classification scheme of Marschner (Marschner, Horstxe2x80x94Mineral Nutrition of Higher Plants 2nd edition Academic Press, 1995). In addition to coating of urea to reduce nitrogen losses, the volatility inhibiting aqueous compositions can be used for dust control purposes when formulating products from granular fertilizer substrates.
The invention relates to processes for preparing coated granular fertilizer products containing urea as the primary nitrogen source in which the volatility or tendency to release nitrogen as ammonia is reduced. It has been discovered that an aqueous borate solution prepared by the neutralization of boric acid with an amino alcohol such as ethanolamine or triethanolamine will reduce the nitrogen loss from soil applied urea when the borate solution is coated onto the surface of granular urea. In a related discovery, it has been found that an aqueous borate solution containing complex ions of a divalent metal such as copper or zinc with ethanolamine can be used to prepare coated urea products with reduced nitrogen losses. The two volatility inhibiting borate mixtures (amino alcohol borate solution, and borate solution containing complex ions of a divalent metal with ethanolamine) can be used to build a number of useful fertilizer compositions that exploit the ability of these compounds to reduce nitrogen losses from soil applied fertilizer materials subject to volatilization.
The volatility reduced urea containing product compositions are prepared from an existing urea granule, a volatility inhibiting aqueous borate solution and if desired other macronutrients and/or micronutrients in the form of fine powders. When other plant nutrients (macronutrients and/or micronutrients) are used to prepare coated urea fertilizer products then the borate solution acts as both a volatility inhibiting agent and a binding agent to hold the additional nutrients onto the granular urea surface. Fertilizer compositions can be built from granular urea that will satisfy specific site requirements with the benefit that nitrogen losses from transformation of urea into ammonia are reduced by the volatility inhibiting borate solution binding agent. The compositions can be used to reduce the nitrogen losses on soils having different pH values making it possible to target compositions for different soil pH values.
Granular urea (46-0-0) is a commercially available product used as the base substrate for building the fertilizer products of the invention. The preferred granular urea has a particle size greater than 2 mm.
The aqueous volatility inhibiting borate solution required to practice the invention can be one of several compositions. The simplest volatility inhibiting borate compositions are prepared by dissolving an amino alcohol such as ethanolamine or triethanolamine in water and then adding boric acid to the aqueous amino alcohol solution. For these aqueous volatility inhibiting mixtures the molar ratio of boric acid to amino alcohol should be above 2.5:1. The best volatility inhibition is obtained when volatility inhibiting borate solution contains above 5% boron and has a final solution pH below 8. When ethanolamine is used a molar ratio of boric acid to ethanolamine of 4.1:1 will permit a borate solution to be prepared which contains 6.2% B with a pH below 8. When triethanolamine is used a 6.2% B solution can prepared with a molar ratio of 2.8:1, which has a pH below 8. The volatility inhibiting ability of the solution is related to the boron content and higher boron contents are needed on higher pH soils.
A borate salt is formed from the volatility inhibiting solution when water is removed from the volatility inhibiting solution. Thus, when the borate solution is used to coat urea and the water is subsequently removed a borate salt is present on the urea surface. If ethanolamine was reacted with boric acid and the water is removed, ethanolamine borate can be obtained. Because of the well known tendency of borates to polymerize in aqueous solution to form polyborates, the borate salt may include some polyborate salts of ethanolamine. Likewise diethanolamine and triethanolamine would yield diethanolamine borate (polyborate) or triethanolamine borate (polyborate). Since it may be difficult to determine which polyborates are present in the volatility inhibiting solutions the salts which form will be described as amino alcohol borates (e.g. ethanolamine borate) and the urea surface will be considered as coated with a mixture of amino alcohol borates.
A second group of volatility inhibiting borate solutions can be prepared from the solution which contains complex ions formed by reaction of ethanolamine with divalent metal. The term complex ion refers to a polyatomic ion formed when a metal ion in solution reacts with a Lewis base (ligand) and 1 or more coordinate covalent bonds form between the metal ion and the ligand. Within the context of this invention, the ligand molecule is an amino alcohol such as ethanolamine. A single ligand molecule may form multiple complex ion structures with the same metal, thus it is possible to have more than one complex ion of a metal ion and a ligand present together.
The complex ion believed to be present in the borate solutions of the invention are those formed by reaction of 4 moles of ethanolamine and 1 mole of the metal ion (copper (II) or zinc). The resulting complex ions have formulas of [Cu(C2H7ON)4]2+ or [Zn(C2H7ON)4]2+, here C2H7ON is the molecular formula for ethanolamine. The aqueous anions which are present in the solution include the anion of the metal salt used and borate anions (polyborate anions).
More than one complex ion involving copper (II) or zinc and ethanolamine may be present in the borate containing aqueous mixtures of the invention although it is believed that the initial complex ion prepared involves four moles of ethanolamine and the metal ion. Thus, the terminology borate solution of complex ions of copper (II) or zinc will be used when referring to these solutions. The abbreviation borate [MEA Cu] or borate [EA Cu] will be used when needed to refer to a borate solution that contains complex ions formed by reaction of ethanolamine and copper (II). The abbreviation borate [MEA Zn] or borate [EA Zn] will be used when needed to refer a borate solution that contains complex ions formed by reaction of ethanolamine and zinc. In addition, the boron and copper (II) or zinc contents of the solutions will be stated.
To prepare the borate solutions containing the complex ions of copper (II) or zinc a water soluble salt of copper or zinc is first dissolved in water and then ethanolamine is added to form the complex ion. Any soluble salt of copper (II) or zinc such as an acetates, chlorides, nitrates or sulfates could be used for preparing the solutions. Acetates, chlorides or sulfates are more desirable due to potential reactions of the nitrate ion with organic substances. An excess of ethanolamine to metal ion is required. A molar ratio of ethanolamine to copper (II) of at least 8:1 (preferably 10:1 or higher) is needed to ensure stability when boric acid is added to the aqueous solution of the complex ion of copper (II) and ethanolamine. A molar ratio of ethanolamine to zinc of 10:1 preferably 12:1 or higher) is needed to ensure stability when boric acid is added to the aqueous solution of the complex ion of zinc and ethanolamine. Heat is liberated when the ethanolamine is added to aqueous metal ion solution and the pH of the solution increases. Hydroxides which form are typically dissolved by ethanolamine and the final pH is typically alkaline (above 8.5). The excess of ethanolamine is needed to ensure that an acceptable molar ratio of unbound amine to boric acid is present when boric acid is added to the solution.
To complete the preparation of the volatility inhibiting borate solution containing the complex ions of copper (II) or zinc with ethanolamine, boric acid is added. The maximum boron content is near 6.5%. The maximum copper (II) or zinc content depends upon the boron content of the mixture and at a boron concentration of 6.2% a borate solution containing 2% copper (II) or zinc is possible from the sulfate salt of each metal. A stable solution containing 5.4% B and 4.6% Cu is possible using copper (II) chloride. Preferably the borate solution containing the ethanolamine metal complexions will have at least 6% B and least 1% of the metal ion. The volatility inhibiting ability of the borate solutions containing the complex ions of ethanolamine with copper (II) tends to increase as the copper (II) content increases if the boron content is constant. The volatility inhibiting borate solutions containing complex ions of zinc with ethanol amine are slightly more effective on higher pH soils than the borate solution containing the complex ions of copper (II) with ethanolamine.
When water is removed from the volatility inhibiting borate solution containing complex ions of a divalent metal with ethanolamine, then a salt containing the complex ion will be present. For example, if copper (II) sulfate is used in preparation of the complex ion containing solution and then water is removed the salt would have the formula [Cu(C2H7ON)4]SO4 where C2H7ON is the molecular formula for ethanolamine. The [] in the formula was included to indicate that the complex ion structure remains. Because the volatility inhibiting solutions have borate anions present the complex ion salts formed when water is removed could include borates or polyborates. When the borate solution containing the complex ions is used to coat urea and water is removed then the surface of the urea will have a mixture of borate salts and other complex ion salts of the divalent metal used. Thus, the urea surface will be considered as coated with a mixture of borate salts and complex ion containing salts.
Borate solutions have a well known tendency to crystallize at low temperatures. Ethylene glycol can be added to both lower the crystallization point and lower the solution pH. Sorbitol will lower the solution pH but will not protect zinc containing mixtures from precipitation when frozen. Copper containing mixtures that contain sorbitol will freeze; however, the mixtures will dissolve when thawed. Adding chelating agents such as citric acid or glucoheptonates will help to stabilize the zinc containing mixtures; however, they appear to counteract the volatility inhibiting ability of the solutions.
Coated urea products of the invention without added macronutrients and/or micronutrients can be formed from the volatility inhibiting borate solution and granular urea. Granular urea is mixed with a desired quantity of the volatility inhibiting borate solution and the wetted granules are allowed to dry until a free flowing product is obtained. The length of drying time depends upon the amount of volatility inhibiting borate solution used to wet the urea granule surface.
To shorten the production time, the wetted urea granules may be dried by well known drying techniques. These techniques can be used provided the temperature of the urea granule wetted with the volatility inhibiting borate solution remains below the melting point of urea (132xc2x0 C.). Preferably, the temperature should be less than 70xc2x0 C. to prevent the formation of phytotoxic biuret.
An alternate approach to obtaining a free flowing granular product from the urea granules wetted with the volatility inhibitor borate solution involves forming a dry surface coating using a flowability aid. Methods for forming the solid coating are described in the sections which follow. The materials which can be used as flowability aids include clays, insoluble phosphate containing minerals and silica and gypsum. The only requirement for forming the coating is that material selected be in the form of a fine powder. Clays have widely been used in the fertilizer industry to improve flowability of products which may contain moisture or hygroscopic products and would be preferred for this purpose due to their low cost.
Coated urea products of the invention which contain macronutrients and/or micronutrients are prepared from granular urea, the volatility inhibiting borate solution (amino alcohol borate or borates containing complex ions of copper (II) or zinc with ethanolamine), and a source of the additional plant nutrients in the form of fine powders. The term fine powder is used to indicate a solid material of which at least 90% will pass through an opening of 149 xcexcm. The volatility inhibiting borate solution acts as both a volatility inhibitor and a binding agent to hold the additional nutrients to the surface of the urea granule. If the additional nutrient material cannot be obtained commercially in the form of a fine powder then it must be pulverized to meet the particle size specification. Equipment is commercially available which is capable of pulverizing the oversize raw material including hammer mills, pin mills, roller mills, etc.
The additional macronutrient sources available for forming the surface coating upon the urea granule with the volatility inhibiting borate binder solution are commercially available phosphate or sulfate salts and mixtures of the same. For example, the monovalent phosphates such as ammonium dihydrogen phosphate (MAP) and potassium dihydrogen phosphate can be used to prepare a phosphate coated urea. In the case of commercially available fertilizer grade MAP, a solution (5% w/v) formed when the MAP is added to water should be less than 5. In addition to the phosphate salts indicated, calcium dihydrogen phosphate (triple superphosphate, TSP) may be used in combination with MAP, however, it will from a wet mass when used alone. When MAP and TSP are used together to form the phosphate coating, the weight ratio of MAP to TSP should be 4:1 or higher. Exemplary sulfate salts that may be used to supply one or more of the additional nutrients for the surface coating include gypsum (calcium sulfate dihydrate), potassium sulfate and potassium magnesium sulfate (sulfate of potash and magnesiaxe2x80x94langbeinitexe2x80x94K2SO4.2 MgSO4; MgSO4 K2SO4.6H2O). Epsom salts (MgSO4.7H2O) forms a wet mass when combined with urea and is unusable in the invention. It is possible to form mixtures of the sulfate salts and a monovalent phosphate salts to form a urea particle coated with additional nutrients containing sulfur and phosphorus in addition to the cation present in either the sulfate or phosphate salts. Ammonium sulfate may be used to supply sulfur as well as additional nitrogen provided the pH of the volatility inhibiting borate binder solution is adjusted to less than 7. Wettable sulfur powder may be used to provide sulfur singly or in combination with other phosphate or sulfate salts.
The micronutrient sources available include the sulfate, nitrate, chloride or acetate salts of copper, iron, manganese and zinc. Mixtures of the indicated salts may be used to supply more than one micronutrient. Boron (in excess that present in the volatility inhibiting borate binder solution) may be added as boric acid (preferred) or a soluble borate salt (ammonium, potassium or ammonium) such as sodium borate (including the metaborates and polyborates and their hydrated forms).
The micronutrient molybdenum is generally required in such small quantities that it can be mixed with the volatility inhibiting borate binder solution to ensure a uniform distribution of the molybdenum. Ammonium, potassium or sodium molybdate are acceptable sources of molybdenum.
The coating step of the invention using urea, a volatility inhibiting borate solution and the additional nutrients previously indicated may be accomplished in more than one manner.
In the first method, granular urea is wetted with the volatility inhibiting borate binder solution by mixing the two materials until the urea granule surface appears damp. The two materials may be mixed in any type of mixing equipment a nd the time required will vary depending upon the type of mixer used; however, the times are usually less than 3 minutes. The final product comprising urea, the volatility inhibiting borate solution and additional nutrients is formed by adding the additional nutrients in the form of fine powders to the urea wetted with the volatility inhibiting borate binder. If more than one additional nutrient source is to be added then powders must be premixed before adding them to the urea granules wetted with the volatility inhibitor borate binder. The mixture of urea wetted with the volatility inhibiting borate binder and additional nutrients is then mixed until a free flowing product is obtained. When mixing is continued too long, the surface coating may be transferred between product granules leaving some product granules with little or no coating. If the mixing time is too short, the powder will not be distributed evenly and the product will have a grainy appearance. Typically, the mixing times after addition of the powders are less than 6 minutes.
A planetary mixer typically used for bread making is very suitable for preparing laboratory size samples. This mixing equipment allows compositions to be quickly evaluated for incompatibility. For larger samples, a tumbling mixer such as that used for preparing mortar mixes or concrete mixing in small batches is suitable. For commercial quantities, equipment used in the fertilizer or pharmaceutical industry for tumble growth agglomeration and coating were found to be suitable for making the products containing urea, volatility inhibiting borate solutions and additional nutrients.
To properly prepare the products composed of urea, volatility inhibiting borate binding solutions and additional nutrients, care must be taken to ensure a proper ratio of urea/binding solution and additional nutrients. It is well known in coating or tumble growth agglomeration, that when too much binding agent is used the products will be wet and have little strength. If too little binding agent is used the resulting products may contain dust. Typically a weight ratio of dry fine powder supplying additional nutrients to volatility inhibiting borate binding solution in the range of 8:1 to 12:1 will give satisfactory products.
The alternate approach to forming the coated urea products from urea, a volatility inhibiting borate binding solution and additional nutrients involves first forming a mixture of dry ingredients then spraying the volatility inhibiting borate solution binding agent into the dry mixture to cause the particles to agglomerate. The volatility inhibiting borate binder solution is added in the form of a spray or mist to cause the fine powders containing desired macronutrients and/or micronutrients to adhere to the surface of the urea granule. Any spraying equipment producing droplets of the volatility inhibiting borate binder solution will be satisfactory.
Any mixing equipment that produces a tumbling bed or mechanically fluidized bed of the mixture of urea granules and desired fine powders of additional nutrients will be satisfactory. The rotating drum mixers are typically used in the fertilizer industry for granulation of fertilizer products and work by producing a tumbling bed of particles. The rotating drum mixers are better for continuous production of product, which is the preferred method of practicing the coating of urea with additional nutrients of the invention. In the continuous process, the urea granules and fine powders are introduced at one end of the mixer and as the bed tumbles the volatility inhibiting borate binder solution is sprayed onto the tumbling bed of particles. The spray of volatility inhibiting borate binder solution results in the additional nutrient supplying fine powders to adhering to the urea surface. The spray of volatility inhibiting borate binder solution can be repeated at multiple points along the length of the rotating drum to ensure complete binding of the fine powders to the surface of the urea granule.
Mechanically fluidized beds involve the use of series of paddles or plows to cause the urea granules and desired additional nutrient supplying fine powders to be constantly suspended within the mixing vessel. As the suspended particles rotate inside the vessel the volatility inhibiting binder is sprayed into the particle suspension. The binder solution causes the particles to adhere together. For this type of mixing equipment the particles initially formed after all binder has been introduced may have a grainy surface coating. The grainy surface coating will disappear if mixing of the particles is continued. The required mixing times were observed to be about 2 times the length of time needed to complete the spraying of the binder into the particle suspension inside the mixing vessel.
The selection of the aqueous volatility inhibiting borate-containing solution (amino alcohol borate or borate containing complex ions of a divalent metal with ethanolamine) for the coating of urea (with or without additional nutrients) can be made based upon local requirements. While many of the examples which follow use the borate solution containing complex ions of copper (II) with ethanolamine because of its better low temperature stability; any one of the volatility inhibiting borate solutions could be used depending upon local conditions. If high boron content is needed then it would be preferable to use an ethanolamine borate solution. If zinc is needed then a borate solution containing complexions of zinc with ethanolamine should be used. Higher viscosity compositions may work better in warm weather and the triethanolamine borate solution could be used under those conditions.
Coating encompasses one of two ideas: the distributing of a liquid coating agent onto the surface of a solid substrate; or the addition of a layer of fine powders onto the surface of a solid substrate. For the second type of coating technique a liquid binding agent may be needed to permit adhesion of the fine particles to the granular surface. In the first case the final particle will often be the same size as the pre-existing granule and will have acquired new properties such as hardness, water impermeability or in our case reduced volatility. In the second case, the particle may be larger in size and the new properties result from properties of the binding agent and the choice of fine powders selected for the surface layer. In our invention the binding agent acts as a volatility control agent and the fine powders act as plant nutrient supplying substances.
Successful coating requires adhesion of materials to an existing particle. The adhesive forces between particles can include physical phenomena and/or chemical reactions where stable chemical bonds between the substrate and added materials form. The physical phenomena that result in adhesion are Van der Waals bonds, ionic bonds and hydrogen bonding. When physical phenomena are involved in the adhesive mechanism a combination of interacting forces often result in greater strength than a single force. Thus, coating agents/binding agents for physical adhesion are more desirable if they support multiple adhesion mechanisms.
In the present invention, physical adhesion mechanisms are believed to dominate. The volatility inhibiting binding agent (coating agent) contains a chemical group capable of ionic interactions that involves a borate anion and an amino alcohol ammonium ion or a borate anion and a mixture of cations that includes the cation formed by ethanolamine metal complex and the ammonium ion of ethanolamine. In all cases the hydroxyl group of the amino alcohol acts as a hydrogen bond forming structure that can adhere to the urea particle via the hydrogen bonds.
The binding mechanism(s) could be supported by a number of other chemical combinations provided that the binding agent has the ability to reduce the nitrogen loss from urea when used in a coating process. The apparent minimal requirement for a binding agent of the invention is a group capable of forming an ionic borate structure and a hydrogen bonding group. In ethanolamine the borate forming structure is the amino group and the hydrogen binding agent is the alcohol group. Any water soluble amino alcohol such as diethanolamine and triethanolamine, the amino propanols and the amino butanols have both a borate forming and hydrogen bonding group.
For the volatility inhibiting borate solution containing the ethanolamine complex of a divalent metal the borate ion is formed when the alkaline solution containing the ethanolamine complex of the metal reacts with boric acid. The hydrogen bonding group is in the case of ethanolamine the primary alcohol group. Ethanolamine is suited because of its large formation constant when forming metal complexes. Stability constants for the formation of complex ions are often expressed in logarithmic form (See Stability Constants of Metal-Ion Complexes (Special Publication SP No. 17) published by The Chemical Society (Burlington House London WIV OBN, 1964) and Stability Constants of Metal-Ion Complexes Supplement No 1 (Special Publication SP No. 25) published by The Chemical Society (Burlington House London WIV OBN, 1971). The stability constants (as logarithms) for complexes with copper (II) and ethanolamine involving 4 molecules of ethanolamine are 16.48 (SP No. 17) and 15.44 (SP No. 25). The stability constants (as logarithms) for complex ion of copper (II) with diethanolamine involving 4 molecules of diethaolamine are 16.00 (SP No. 17) and 14.6 (SP No. 25). The stability constants (as logarithms) for complex ion of zinc with ethanolamine involving 4 molecules of ethanolamine are 9.2 to 9.4 (SP No. 25) and the stability constant (as a logarithm) for the complex ion of zinc with diethanolamine involving 4 molecules of diethanolamine is 9.11 (SP No. 25). The ability to prepare borate solutions from a solution of the complex ion of copper (II) or zinc ion with an amino alcohol is limited to those amino alcohols which form complex ions with copper (II) or zinc that have stability constants greater than 109 (logarithm of stability constant greater than 9) for the complex ions having 4 moles of amino alcohol per mole of metal. Ethanolamine and diethanolamine are preferred.
Polyols are an additional class of substances capable of forming borates which also contain a hydrogen bonding group. The polyols include sugars (glucose, fructose, ribose, and sucrose) and sugar alcohols (sorbitol, mannitol, and glycerol) and sugar acids (glucoheptonates, and gluconates). These substances are known to form complexes with boric acid and in fact increase the acidity of boric acid. Th e complex involving the polyol with boric acid joins two adjacent hydroxyl groups of the polyol in a borate anion. A hydrogen ion is released as the borate anion forms. The remaining hydroxyl groups of the polyol provide the hydrogen bonding groups.
The primary weakness of the polyol group is the low amount of boron which is present in the resulting aqueous solutions. The borate complex of many sugars is unstable and boric acid will precipitate easily. Sugars are nutrient source for soil bacteria and may increase microbial attack upon the urea particle. The borate complexes of some commercially available sugar acids are typically very viscous and difficult to handle. We, also, have observed that when mixtures of some commercially available sugar acids with amino alcohol borates (e.g. ethanolamine borate) are prepared that the resulting mixtures lose their ability to reduce nitrogen losses when coated onto granular urea. Thus, the sugar alcohols would be preferred materials for forming a volatility inhibiting binder mixture. We have found that the borate complex of sorbitol will acts as a good binding agent when preparing ammonium sulfate coated urea fertilizers. When a plant species is particularly sensitive to boron, the low boron content of the sorbitol boric acid complex should permit manufacture of urea coated fertilizers of limited boron content.
The ability of a binding agent to effectively hold two or more particles together can depend upon the viscosity of the binding agent. Coalescing particles must have sufficient time in contact with each other for the adhesive forces to develop before attrition forces due to mixing separate the particles. The quality of products resulting from a coating (agglomeration) technique are often improved by binding agents with high viscosity.
The quality of the coated products prepared from the amino alcohol borates is believed to vary with the viscosity of the volatility inhibiting binding solution. Products composed of urea, a volatility inhibiting binding agent and additional nutrient supplying fine powders require longer mixing times to achieve adequate coating when the viscosity of the volatility inhibiting binding agent is around 30 cps (measured at room temperature). Mixing times are much shorter and the coverage of the urea granules is generally better when the viscosity of the volatility inhibiting binding agent is over 60 cps measured at room temperature).
The viscosity of the volatility inhibiting binding mixtures of the invention (amino alcohol borates and borate solutions containing the ethanolamine complexes of copper (II) or zinc) depends upon both boron content of the mixture and final solution pH. In general the lower the pH and boron content the lower the viscosity. To improve performance during coating particularly when additional nutrient supplying substances are needed, a volatility inhibiting binding agent can be selected which has a higher viscosity such as diethanolamine or triethanolamine. Mixtures of ethanolamine with diethanolamine (preferred if the borate solution contains a complex ion of a metal with an amino alcohol) or triethanolamine can be used to increase the solution viscosity. Alternatively, commercially available materials known to act as thickening agents such as glycerol, low glucose content polysaccharides, polysaccharide gums could be added as viscosity control agents. The additional viscosity control agent must not react with copper (II) in alkaline solution which is characteristic of corn syrups, thus, corn syrups are not preferred viscosity enhancing agents when used with the volatility inhibiting mixture contains copper (II).